Are you either excited about GHC 7.2.1 or do you require it because you upgraded to Mac OSX Lion (where 7.0.* is plagued by linker errors)? If so, then you might be interested in the procedure outlined in this gist. GHC builds fine even with the broken 7.0.* as a base, but cabal-install has been recalcitrant.
Since the process of patching and updating is a little fiddly, I’ve taken the expense of hosting the patched cabal-install ready for you to download, untar, and run "sh bootstrap.sh" upon.
Make sure before you start to move your old 7.0.* ~/.cabal and ~/.ghc directories somewhere else; they can break the boostrapping.